When infection with a virus such as an influenza virus occurs, a virus elimination reaction due to natural immunity proceeds, a specific immune response is subsequently induced, and a virus elimination reaction proceeds.
In the specific immune response, virus in a body fluid is eliminated by a neutralizing antibody, and virus within a cell is eliminated by a cytotoxic T lymphocyte (CTL). That is, the CTL specifically recognizes a virus antigen (CTL epitope) consisting of 8 to 11 amino acids presented in an HLA class I molecule on the surface of an infected cell, and eliminates the virus by damaging the infected cell. Identifying such a virus-specific CTL epitope is therefore important for preparing preventive and therapeutic vaccines for the virus.
A technique of this kind is known from Patent Publication 1. Patent Publication 1 states that an oligopeptide formed from a specific amino acid sequence has the property of binding to an HLA.    [Patent Publication 1] Japanese Patent Application Laid-open No. H8-151396 (1996)